The invention relates to a method of positioning the operating region of a tool component at a pre-determined, random grid element in a nominally regular grid of nominally identical grid elements, whereby the positioning occurs with the aid of an adjusting mechanism, comprising the steps of:                a. finding an initial position reference in the grid;        b. performing a displacement of the tool component with respect to the grid;        c. subsequently finding a position reference in the grid;        d. testing if the pre-determined grid element is within a pre-determined distance of the operating region of the tool component, and;        e. depending upon the result of the test, repeating step b, and, if necessary, repeating steps c and d.        
The invention also relates to an apparatus for performing the method according to the invention.
Such a method is employed, for example, in the semiconductor industry in defect analysis, where there is a desire to inspect a specific element, such as a memory cell, in a grid of nominally identical memory cells in a semiconductor circuit, with the aid of, for example, a Scanning Electron Microscope (SEM). Such a semiconductor circuit can already be broken out of a wafer, or it can still constitute a part of the wafer.
In the case of a SEM, a focused electron beam inspects a sample, such as a semiconductor circuit. At the location where the electron beam impinges upon the sample, secondary electrons inter alia are emitted, which are detected using a Secondary Electron Detector (SED). In this manner, (position-dependent) information is obtained, which is subsequently made available in the form of an image. The size of the focus of a SEM is nowadays 1 nm, for example, so that images with a very high resolution can be made herewith.
In the semiconductor industry, so-called wafers with a cross-section of, for example, 30 cm are processed into ICs. At a certain point, a wafer is broken up into tens or even hundreds of ICs, whereby countless semiconductor elements are present on each IC. These semiconductor elements often form grids of nominally identical structures on such an IC, as is, for example, the situation in the case of semiconductor elements in the form of memory cells. So as to reduce production loss, it may be desirable to inspect specific semiconductor elements.
The desire to inspect a specific element, such as a specific memory cell, can arise, for example, from an electrical test, in which it transpires that the properties of the element concerned deviate from the specifications of other elements. By inspecting such a specific element, one can obtain insight as to the cause of the deviation, which can lead to process and/or production improvements and, accordingly, to reduced production loss.
Memory cells such as used nowadays in the semiconductor industry often occupy an area of less than a square micron, and are organized into grids, in which, for example, 1000*1000 memory cells are arranged. So as to be able to inspect a specific memory cell, e.g. with a SEM, the operating region of the SEM (the image field) will have to be displaced to this specific memory cell. To this end, it is first necessary to locate an easily recognizable point, such as a corner point of the grid, after which one can displace to the memory cell to be inspected.
In the method described above, an image of the semiconductor circuit as made available by the SEM is continuously depicted on a display. The semiconductor circuit, which is mounted on a moving mechanism such as an x-y table, can be displaced using a joystick, for example. First, the image field is displaced to a grid element at a known position, such as a corner point of the grid. From this known position, one knows in advance by how many rows and columns one has to displace so as to arrive at the element to be inspected. Subsequently, the semiconductor circuit is displaced with the aid of the joystick, whereby the displacement of the semiconductor circuit on the display is simultaneously followed by eye. By now counting—during displacement—by how many rows and columns the image displaces across the semiconductor circuit, the pre-determined position is eventually reached.
A disadvantage of the known method is that there is a great risk of counting errors, e.g. as a result of interruptions or disturbances in concentration of the personnel performing the method. A counting error can easily be made, or one can loose count, after which the whole process will have to be performed anew. One must realize in this context that the movement of the x-y table at this magnification, whereby elements of less than 1 μm2 are imaged, occurs in a relatively abrupt manner.
It should be noted that the elements do not necessarily have to be ordered in a rectangular grid, but that this can also be a skewed grid or, for example, a grid in the form of a honeycomb. Such grid forms increase the risk of counting errors.